Polymeric membranes may be prepared by the phase inversion technique which commences with the formation of a molecularly homogeneous, single phase solution of a polymer in a solvent. The solution is then allowed to undergo transition into a heterogeneous, metastable mixture of two interspersed liquid phases one of which subsequently forms a gel. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermal precipitation.
The quickest procedure for forming a microporous system is thermal precipitation of a two component mixture, in which the solution is formed by dissolving a thermoplastic polymer in a solvent which will dissolve the polymer at an elevated temperature but will not do so at lower temperatures. Such a solvent is often called a latent solvent for the polymer. The solution is cooled and, at a specific temperature which depends upon the rate of cooling, phase separation occurs and the liquid polymer separates from the solvent.
All practical thermal precipitation methods follow this general process which is reviewed by Smolders et al in Kolloid Z.u.Z Polymer, 43, 14-20 (1971). The article distinguishes between spinodal and binodal decomposition of a polymer solution.
The equilibrium condition for liquid-liquid phase separation is defined by the binodal curve for the polymer/solvent system. For btnoda/decomposition to occur, the solution of a polymer in a solvent is cooled at an extremely slow rate until a temperature is reached below which phase separation occurs and the liquid polymer separates from the solvent.
It is more usual for the phases not to be pure solvent and pure polymer since there is still some solubility of the polymer in the solvent and solvent in the polymer, there is a polymer rich phase and a polymer poor phase. For the purposes of this discussion, the polymer rich phase will be referred to as the polymer phase and the polymer poor phase will be referred to as the solvent phase.
When the rate of cooling is comparatively fast, the temperature at which the phase separation occurs is generally lower than in the binodal case and the resulting phase separation is called spinodal decomposition.
According to the process disclosed in U.S. Specification No. 4,247,498, the relative polymer and solvent concentrations are such that phase separation results in fine droplets of solvent forming in a continuous polymer phase. These fine droplets form the cells of the membrane. As cooling continues, the polymer freezes around the solvent droplets.
As the temperature is lowered, these solubilities decrease and more and more solvent droplets appear in the polymer matrix. Syneresis of the solvent from the polymer results in shrinkage and cracking, thus forming interconnections or pores between the cells. Further cooling sets the polymer. Finally, the solvent is removed from the structure.
Known thermal precipitation methods of porous membrane formation depend on the liquid polymer separating from the solvent followed by cooling so that the solidified polymer can then be separated from the solvent. Whether the solvent is liquid or solid when it is removed from the polymer depends on the temperature at which the operation is conducted and the melting temperature of the solvent.
True solutions require that there be a solvent and a solute. The solvent constitutes a continuous phase and the solute is uniformly distributed in the solvent with no solute-solute interation. Such a situation is almost unknown with the polymer solutions. Long polymer chains tend to form temporary interactions or bonds with other polymer chains with which they come into contact. Polymer solutions are thus rarely true solutions but lie somewhere between true solutions and mixtures.
In many cases it is also difficult to state which is the solvent and which is the solute. In the art, it is accepted practice to call a mixture of polymer and solvent a solution if it is optically clear without obvious inclusions of either phase in the other. By optically clear, the skilled artisan will understand that polymer solutions can have some well known light scattering due to the existence of large polymer chains. Phase separation is then taken to be that point, known as the cloud point, where there is an optically detectable separation. It is also accepted practice to refer to the polymer as the solute and the material with which it is mixed to form the homogeneous solution as the solvent.
There are several characteristics in the morphology of a membrane that can describe what is observed when phase inversion membranes are scrutinised under an electron microscope. The morphological characteristics may be described with the terms symmetry, homogeneity and isotropy.
Symmetry means that one half of the structure is the mirror image of the other half. The device about which a membrane is symmetrical is a plane or surface half way between the two faces of the membrane. In membrane science, the term is often incorrectly used to mean homogeneous. Homogeneous means simply that the membrane has a uniform structure. In chemistry, the term "homogeneous", when ascribed to a substance, means that it has uniform structure or composition.
Isotropic means that the membrane has equal properties in all directions. The word isotropic comes from biology where it is means a tendency for equal growth in all directions.
The opposite of these terms are often used, namely--asymmetric, non-homogeneous and anisotropic. Anisotropic is often incorrectly understood to mean asymmetric or non-homogeneous. Anisotropic more currently describes how a morphology develops rather than the nature of the morphology.
In membrane science, the meaning of the above words has been refined by technological development. Prior to about 1960, phase inversion membranes were isotropic or only slightly anistropic. About that time, membranes with more inhomogeneity were developed.
Taking a vector from one face of a membrane to another, there are two types of inhomogeneity of importance known in membrane science as skinning and anisotropy.
Skinning is used as a synonym for asymmetry, and refers to a membrane having a relatively thin dense layer at one surface of the membrane with a relatively thick porous substructure throughout the remainder of the membrane. The first skinned membrane made by phase inversion is described in U.S. Specification No. 3,133,132 which discloses the solvent intrusion method of phase inversion.
In addition to the terms used to describe how one region of porous membrane is related to another, there are more specialised terms used in describing the shapes of the pores themselves.
Membrane scientists use the word structure when referring to the shapes of pores, cells, alveoli and other void shapes found within the membrane. The structure can be described as granular, spongy, reticulate, or lacey. The voids can be described as cells, or cells with interconnecting pores, and larger cavities can be described as macrovoids.
When viewed under an electron microscope, granular structures are characterised by polymer balls roughly spherical in shape which appear to be fused together as if sintered. Granular structures are not generally desirable in microporous membranes because the porosity and mechanical strength are both lower than other types of structure.
A spongy structure is characterised by roughly spherical cells connected by roughly cylindrical conduits or pores. Such a structure is disclosed in U.S. Specification No. 4,519,909.
A reticulate structure is characterised by a netlike appearance.
On the other hand, the polymertc material which forms the substance of a lacey structure can be described as multiply connected strands of polymer, with each connection point having only slightly larger dimensions that the cross-section of the strands. The strands have a length substantially larger than the largest cross-sectional dimension, and the shape of the cross-section of the strands varies from strand to strand as well as along the strand. The shape of the cross-section of the strands can be described as round or ensiform, circular or oval. The strands may have grooves or furrows, or even appear to be like a multiplicity of coalsaced filaments.
All of the above structures are bicontinuous in the solid state in that every part of the polymer is connected to every other part of the polymer, and every cavity is connected to every other cavity in an intermingled porous network of polymer and cavity.
As well as the above structures, interposed upon granular, spongy and lacey structures there can be cavities of substantially larger dimensions than those described earlier, and these cavities are referred to as macrovoids. Macrovoids which are elongated in shape are called finger voids, and macrovoids which are rounder in shape are called alveoli.
Macrovoids are, by definition, completely surrounded by the microporous structure of the membrane.
Several membranes made of polyvinylidene fluoride have been cited in the literature. Most are sheet membranes which are made by the common process of non-solvent (or poor solvent) intrusion to cause Gelation or phase inversion.
For example, U.S. Pat. No. 3,642,668 discloses dimethyl sulfoxide (DMSO) or dimethyl acetamide (DMAc) as the solvent for polyvinylidene fluoride when casting a membrane onto a support structure, immediately followed by immersion in a non-solvent bath, typically methanol.
Japanese Patent No. 51-8268 uses cyclohexanone as a solvent for polyvinylidene fluoride. The solution is heated and then cooled during which time the solution passes through a region of maximum viscosity. The membrane is cast when the viscosity of the solution is decreasing.
European Patent No. 223,709 discloses a mixture of acetone and dimethyl formamide (DMF) as a preferred solvent although all the usual standard or active solvents such as ketones, ethers such as tetrahydrofuran and 1,4 dioxane, and amides such as DMF, DMAC and DMSO are described. The membrane is formed by coating the polymer solution onto a substrate which is immediately immersed in a poor solvent.
In the process disclosed in U.S. Pat. No. 4,203,847 flat sheet membranes are formed by casting a nearly saturated solution in hot acetone onto a moving belt which then passes into a forming bath containing a mixture of solvent and non-solvent. This produces a thin skinned membrane. U.S. Pat. No. No. 4,203,848 describes the belt and machine used in this process.
U.S. Pat. No. 3,518,332 discloses a flat sheet membrane made by pressing and sintering a mixture of polyvinylidene fluoride particles with particles of a metallic salt and paraffin wax.
U.S. Pat. No. 4,810,384 describes a process wherein polyvinylidene fluoride and a hydrophilic polymer compatible therewith are dissolved in a mixture of lithium chloride, water and dimethylformamide, then cast onto a web and coagulated by passing the film through a water bath. A hydrophilic membrane that is a blend of the two polymers is produced.
U.S. Pat. No. 4,399,035 discloses a polyvinylidene fluoride membrane prepared by casting a dope comprising polyvinylidene fluoride, an active solvent such as DMAc, N-methylpyrrolidone or tetramethylurea and a minor amount of a surfactant or mixture of surfactants into a non-solvent bath, typically water or an alcohol. Polyethylene glycol and polypropylene glycol are used as surfactants and glycerin fatty acid esters are mentioned in the description as being suitable.
U.S. Pat. No. 4,666,607 describes a thermal gelation process. It discloses the use of a quench tube in the form of a U-tube, or a tank with the fibre moving as if in a U-tube, which can be used to produce polyvinylidene fluoride films or hollow fibres by extrusion of a solution comprising the polymer, solvent(s) and a non-solvent above the temperature at which the solution will separate into two phases, advantageously through an air gap into a cooling liquid in the quench tube or tank. In the case of hollow fibres, a lumen forming fluid (which is not a solvent for the polymer) is employed.
Emphasis is placed on the avoidance of stress on the extruded, but still liquid, fibre and the stretch factor (i.e. the ratio of the velocity of the formed, cooled fibre membrane to the velocity of the polymer solution emerging from the forming die) is typically in the region of only 1.33.